Process industry plants, for example, chemical, petrochemical, pharmaceutical, food and other products manufacturing industries, may include, at a field level, locally distributed, decentralized process automation devices, that is, field devices. Such field devices have predefined functions within the plant's process automation system and are involved in an exchange of information associated with the process, the plant, and/or other field devices, with components of a monitoring and control system, and also amongst one another. The field devices may sense information about real world signals such as pressure, flow, level, and temperature. Conventionally, in a process automation system, the transfer of process data from the field devices to the monitoring and control system devices such as programmable logic controllers (PLCs), is carried out in the form of analog current values ranging between about 4 milliamps (mA) and about 20 mA.
FIG. 1 illustrates a conventional equivalent circuit of a two-wire current loop 100 in a process automation system. The two-wire current loop 100 communicatively couples a field device 101 with a receiver 102 via a cable. The field device 101 is, for example, a pressure transmitter in communication with a sensor (not shown) sensing pressure values, in the process automation system. The field device 101 is connected to a power source 103. In a current loop 100, cables may have a varying range of length and may be over 1 km long, due to which, voltage at device terminals 101a and 101b may drop to about 10.5 volts (V) or increase to about 42V based on the length of cable therebetween. Therefore, the power source 103 is a DC power source, for example, including one or more battery sources of 24V or 12V, powering the field device 101, and/or the receiver 102. The receiver 102 is, for example, a programmable logic controller (PLC). The field device 101 converts these pressure values that are essentially the real world signals, into the control signals necessary to regulate the flow of current in the two-wire current loop 100. The loop current is adjusted by the field device 101 to be proportional to the actual measured pressure input. The field device 101 may use a 4 mA output to represent a calibrated zero input, and a 20 mA output to represent a calibrated full-scale input signal. The field device 101 draws its operating power from the loop current flowing therein. This field device 101 modifies the loop current flowing over a cable such that the receiver 102, that is, the PLC 102, receives and measures this modified loop current in order to sense the measurement recorded by the field device 101. The PLC 102 is further connected to monitoring and control system (not shown) which in turn is connected to a host computer in the process automation system.
FIG. 2 illustrates a conventional current output stage 200 of the field device 101 illustrated in FIG. 1. The field device 101 is connected to the current loop 100 via terminals 101a and 101b. The current output stage 200 includes a regulator module 201 including an operational amplifier (OPAMP) configured as a differential amplifier. The regulator module 201 compares two inputs namely, a first input, for example, a set point voltage 203, associated with a sensed value of a real world signal, such as pressure, obtained from a sensor sensing the real world signal and communicating with the field device 101 and a second input, for example, a feedback voltage representing the loop current flowing in the current loop. The input values to be compared are obtained using a resistor divider module 202 including resistors R1 and R2, and a sense resistor Rs. However, the resistors R1, R2, and Rs in the conventional current output section 200 directly affect temperature characteristics thereby affecting accuracy of measurement, and long-term stability behavior of the current output. In some applications, military grade components may be employed to preclude affects associated with temperature characteristics, however, these lead to a drastic increase in the costs associated therewith. The effect of temperature characteristics may be reduced by calibrating the behavior of the resistive components at a manufacturing stage. However, this calibration over temperature is expensive. In addition, the long-term stability of such resistive components is not specified by the manufacturers and suppliers, as this requires a series of tests, thereby leading to increase in the costs. Moreover, the long-term stability of a device cannot be designed using a worst case limitation as the long term stability behavior is designed based on real time application conditions. To increase long-term stability, analog signals may be converted to digital signals before being transmitted to the PLCs, however, this involves additional circuitry leading to higher power consumption. The two-wire current loops may be power limited and required to be efficient in power conditioning, in order to provide safe operation in hazardous environments having a large number of variants. The power may be limited in a range of about 10 milliwatts (mW) to about 40 mW.
Therefore, it is an object of the present disclosure to provide a field device of the aforementioned kind having an analog output stage that regulates an analog output based on an input detected by the field device, without compromising on cost, temperature based measurement accuracy, and long-term stability of the field device.